Chemical-mechanical polishing (CMP) is the process of removing projections and other imperfections from a semiconductor wafer to create a smooth planar surface. The wafer is the basic substrate material in the semiconductor industry for the manufacture of integrated circuits. Wafers are typically created by growing an elongated cylinder or boule of single crystal silicon and then slicing individual wafers from the cylinder. Slicing causes both faces of the wafer to be somewhat rough. Planarization is desirable because the front face of the wafer on which integrated circuitry is to be constructed must be substantially flat in order to facilitate reliable semiconductor junctions with subsequent layers of material applied to the wafer. Composite thin film layers comprising metals for conductors or oxides for insulators must also be made of a uniform thickness if they are to be joined to the semiconductor wafers or to other composite thin film layers.
Planarization is typically completed before performing lithographic processing steps that create integrated circuitry or interconnects on the wafer. Non-planar surfaces result in poor optical resolution of subsequent photolithographic processing steps which in turn hinders high-density features from being adequately printed. If a metallization step height is too large, open circuits will likely be created. Consequently, CMP tools are continually being improved upon with an aim toward controlling wafer planarization.
In a conventional CMP assembly the wafer is secured in a carrier connected to a shaft. The shaft is typically connected to a transporter that moves the carrier between a load or unload station and a position adjacent to a polishing pad. One side of the polishing pad has a polishing surface thereon, and an opposite side is mounted to a rigid platen. Pressure is exerted on a wafer back surface by the carrier in order to press a wafer front surface against the polishing pad. Polishing fluid is introduced onto the polishing surface while the wafer and/or polishing pad are moved in relation to each other by means of motors connected to the shaft and/or platen. The above combination of chemical and mechanical stress results in removal of material from the wafer front surface. One requisite for removing wafer material at a high rate (“removal rate”) and for forming a wafer with high surface uniformity is a uniform distribution of polishing fluid about the polishing surface.
In the case of CMP tools that use a rotating polishing platen and pad, one way that the polishing fluid is supplied to the polishing surface is through one or more delivery outlets that deposit the polishing fluid onto the polishing pad near the wafer leading edge. However, polishing fluid is not efficiently utilized with this type of delivery system. Due to the centrifugal force from the rotating platen the polishing fluid is quickly evacuated from the pad surface and the wasted polishing fluid must be continuously replaced. Visual examination of the polishing pad also reveals that the polishing fluid accumulates at the pad outer edge during polishing. As mentioned above, non-uniform polishing fluid distribution causes poor wafer planarization, and this problem alone necessitates an improved polishing fluid supply mechanism.
Another way that the polishing fluid is supplied to the polishing surface is through a plurality of through-holes distributed about the polishing pad. The polishing pad through-holes are in communication with a supply source via holes or passageways extending through the platen. This “through-the-pad” polishing fluid delivery system is known to provide improved polishing fluid uniformity during polishing. Through-the-pad polishing fluid delivery systems have been successfully used on “non-rotational” type CMP tools having a polishing surface not much larger than the wafer, and which moves in an orbital or reciprocating motion. However through-the-pad fluid delivery has not been shown to provide improved polishing fluid uniformity when used in conjunction with the type of CMP tool incorporating a rotating polishing pad. This is due at least in part to the relative mismatch in wafer and platen diameter. Because the polishing surface is necessarily substantially larger than the wafer in a rotating polishing pad CMP tool, usually more than twice the wafer diameter, some polishing pad through-holes are covered by the wafer that is being polished, while others are left uncovered. The uncovered holes are naturally passages of lesser resistance, and consequently, little if any polishing fluid is delivered directly to the wafer-pad interface during polishing, while large amounts of slurry is wasted through the uncovered holes.
Accordingly, it is desirable to provide a CMP polishing fluid supply mechanism that enables substantially uniform polishing fluid distribution about a pad-wafer interface during polishing on a rotating platen type polishing apparatus. In addition, it is desirable to provide a CMP polishing fluid supply mechanism that efficiently utilizes the polishing fluid. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.